The present invention relates generally to high voltage breakdown devices and more specifically to high voltage breakdown devices in dielectrically isolated islands. Although the invention described with respect to a diode since it is the simplest of the PN junction structures, the invention is also applicable to BJT's, DMOS SCR-GTO's and diffused resistors.
Electrical breakdown of reversed biased junctions occurs when the electric field over some distance exceeds a critical value. In this high field region, any free holes or electrons receive enough energy to generate electron-hole pairs though collision with the semiconductor atoms. These newly generated carriers then generate more carriers in the same way and is known as avalanche multiplication. This results in current many orders of magnitude larger than junction leakage current and increases exponentially with the electric field which is proportional to the applied voltage. The onset of this current defines the breakdown voltage which is a maximum voltage for useful operations.
Since the energy can be imparted to carriers over a distance less than one micron, the analysis will be in terms of peak electric field. If the peak electric field exceeds some critical value (approximately 30-35 volts per micron) anywhere within the device, breakdown will occur. The electric field can be calculated by solving Poison's equation: EQU .gradient..multidot..epsilon.E=.rho. Eq. 1
wherein E is the electric field vector, the .epsilon. is the permittivity of the material and the .rho. is the charge density. Equation 1 is solved as a boundary value problem wherein the boundaries for a semiconductor devices are usually assumed to be field free surfaces of constant voltage. The electric field vector E and voltages V are related by ##EQU1##
The peak electric field E then as a function of applied voltage, the device shape, the doping concentration which determines the charge density and the material which determines the permittivity.
In most cases, the edge of a diffusion defined by a mask results in curvature in the plane of the PN junction. This curvature often causes an increase in electric field which reduces the breakdown voltage. Therefore breakdown usually occurs at the corners of diffusion at a lower voltage then the voltage which could be obtained if the corners were eliminated. The curvature of the junction makes one of the constant voltage boundary surfaces have a smaller surface area than the other. This smaller surface area causes the electric field lines to crowd together and thereby geometrically produces the increase in electric field.
Several structures have been developed which reduce electric field at the edge of a diffusion. These are:
1. large radius of curvature instead of sharp corners PA1 2. guard rings PA1 3. field plates (U.S. Pat. No. 4,713,681 Beasom) PA1 4. equipontential rings (#1-4 discussed in Semiconductor Devices, Chapter 2 by S. Ghandi, Wiley & Sons, 1977) PA1 5. field plates over stepped oxide ("Structural Analysis and Experimental Characteristics of High-Voltage Bipolar Transistors with Shallow Junctions", Sakurai & Ohno, Jap. J. of Applied Phy., Vol. 23, no 4, April 1984, pp 415-419) PA1 6. floating metal rings ("Floating Metal Rings . . . ", Yilmaz, IEEE ED letters, Vol EDL-6, no 11, Nov. 1985, p 600) PA1 7. junction termination extensions ("Multiple-Zone Single-Mask Junction Termination Extension . . . ", Tantraporn & Temple, IEEE tran ED, Vol ED-34, no 10, Oct. 1987, p 2200-2210) PA1 8. shallow surface layer implants (Harris patent file SE-368 Beasom, U.S. Pat. No. 4,975,751 issued Dec. 4, 1990).
Although these prior art solutions have produced high breakdown voltage devices, they have generally required substantial amount of surface area as well as complex masking. The field plates for example require tailored oxide thickness. The guard and floating metal rings need anode to cathode spacing of greater than 100 microns. The breakdown voltage of these devices are effected by the potential of the substrate relative to the island. For substrate bias between that of the anode and the cathode, the cathode potential is the worst case voltage assuming P diffusion into N island.
Thus is an object the present invention to provide a high voltage junction, dielectrically isolated islands which is not process dependent.
Another object of the present invention is to provide a high voltage junction in dielectric isolated islands which is not dependent on oxide thickness and allow smaller spacing ground rules.
An even further object of the present invention is provide a high voltage junction dielectrically isolated islands which is less effected by substrate bias.
These and other objects of the invention are attained by introducing impurities of a second conductivity type into a dielectrically isolated island of a first conductivity type so as to form a first region of the second conductivity type extending to and between a pair of opposed dielectric isolation walls. This shifts the boundaries to the dielectric walls and removes the low breakdown regions. Impurities of the second conductivity type are introduced into the first region to form a second contact region of higher impurity concentration than first region. First conductivity type impurities are introduced into the dielectric island to form a island contact region spaced from the first region. These contacts regions define a current path transverse to the lateral extent of the first region. The first region is formed having edges which diverge, horizontally and vertically to and towards the opposed dielectric walls. The impurities are introduced through an opening in a mask. The mask for the first region is sufficiently close to the opposed dielectric walls such that the impurities that are introduced therethough diffuse to the opposed dielectric side walls. The lateral edge of the mask region is within approximately 0.7 times the vertical depth of the to-be-formed first region from the dielectric side walls.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.